Heat exchange device



June 20, 1961 J. D. CHRISTIAN 2,989,289

' HEAT EXCHANGE DEVICE Filed June 15, 1959 I 4 Sheets-Sheet 1 E rigs E OFF 5L [CK 7 4 l l I A ORNEYS '1 MEI 'M .4 MEM 5 ER OF 11/: mar

June 20, 1961 Filed June 15, 1959 J. D. CHRISTIAN HEAT EXCHANGE DEVICE 4 Sheets-Sheet 2 Joseph D. ChrisT/an EC/(l-IOFF .SL/CK A TTORNEYS a I l A MEMBER OF 77/5 FIR June 20, 1961 J. n. CHRISTIAN HEAT EXCHANGE DEVICE 4 Sheets-Sheet 3 Filed June 15, 1959 INVENTOR. Joseph D. Chr/sf/afl EC/(HOFF t SLICK 4 MEMBER OF THE PIE ATTORNEYS 1,irialiaaf:.2

United States Patent F 2,989,289 HEAT EXCHANGE DEVICE Joseph D. Christian, 480 Potrero, San Francisco, Calif. Filed June 15, 1959, 'Ser. No. 820,555 11 'Claims. (Cl. 257-86) This invention relates to an improved method and apparatus for the continuous production of a calcined gypsum.

There is a widespread commercial demand for calcined gypsum containing a predetermined quantity of water of crystallization, usually about 6.2% and corresponding to calcium sulfate hemihydrate, 2CaSO -H O.

In the past, this material has been produced generally by two different methods, a batch method and a continuous method. In the batch method, the gypsum rock is placed in a pot or kettle and heated by direct firing to 1500-2000" F. The heating is continued until the material has the desired reduced water content. This is determined, not by any chemical method, but by the physical appearance of the material in the kettle; the skill of the operator, therefore, enters into the quality of the final product. It will be obvious that the product may vary as between operators and the apparatus employed.

In the continuous operation, the gypsum rock is calcined in a rotary calciner which is directly fired with a carbonaceous fuel such as oil or gas. Necessarily, some contamination results from the carbon produced during combustion of the fuel used for calcination, the result being that the final product contains black specks which are undesirable. Generally, a calcined product produced by this method will not qualify as a first-grade material.

In both processes, the danger is inherently present under-calcination or over-calcination may occur with the result that some of the dehydrite, CaSO -2N O or the anhydride, CaSO are present. Both these materials are highly undesirable in the product because of the difficulties which they provide upon any subsequently attempted hydration.

In various recently constructed gypsum calciners utilizing helical flights, it is possible to process gypsum in a somewhat more eificient manner than has been possible heretofore but considerable handling of the gypsum is still required, since it is necessary to move the gypsum from one vessel to another whereby to enable it to be subjected to the varying temperatures required throughout the processing operation.

In accordance with the present invention, I provide an apparatus and a method enabling finely divided gypsum rock to be heated continuously under such conditions that the combined water content of the resulting product can be carefully controlled at all times. A further object of the present invention is to provide a novel form of apparatus enabling the residence time of gypsum rock undergoing calcination to be carefully controlled to the end that the desired white calcined product can be produced which can pass directly into the channels of trade.

Still another object of this invention is to provide an apparatus and a continuously operating method wherein gypsum may be subjected to varying temperature conditions without at any time necessitating that the gypsum be removed from one unit and recharged into another or that the direction of movement of the gypsum be changed.

In putting the process into practice, the gypsum rock a Patented June 20, 1961 ice is first reduced in size to the end that the resulting calcined product will be in an acceptable form. Usually, the gypsum rock is first reduced in any suitable type of communication equipment to a state whereat about of the rock will pass through mesh screen. Because communication involves heavy frictional force application to the rock, the temperature will generally rise to a point whereat any free or uncombined water or moisture is driven off from the rock, e.g., to 180 F. If this does not sufiice, the ground rock can be separately dried or it can be processed with free water present. A suitable drying or aridizing solution such as calcium chloride can also be added if desired and is as known in the art.

The product from the communition operation may desirably be sent directly to the calciner wherein the ground rock is heated suitably to drive off the desired amount of water of crystallization. To this end, the rock is subject to temperatures above 212 F. and as high as 400 F. for a period which may vary from 10 to minutes. Utilizing the apparatus of the present invention, it is possible to control the temperature of gypsum passing through any given zone of the structure, a convenient arrangement employing three separate zones, each of which is connected to a heat exchange medium. This makes it possible to provide progressively lower temperatures from the feed to the discharge end of the apparatus. Since it is undesirable to hold the gypsum too long above a given maximum, means of this sort for providing for progressively reduced temperatures as the gypsum approaches the discharge end of the apparatus is particularly important.

The apparatus which is useful in the practice of the process of this invention is particularly 'shown in the drawings in which:

FIGURE 1 is a side elevation, partly in section, through a preferred form of gypsum calciner embodying this invention.

FIGURE 2 is a section taken along the line 2-2 in FIGURE 1.

FIGURE 3 is a section taken along the line 3-3 in FIGURE 1.

FIGURE 4 is a sectional view taken through the base of the apparatus, showing the drive equipment.

FIGURE 5 is a section taken along the line 5-5 in FIGURE 4.

FIGURE 6 is a section taken along the line 66 in FIGURE 5.

FIGURE 7 is a side elevation partly in section of a form of the apparatus embodying certain modifications.

FIGURE 8 is an enlarged sectional side elevation of the manifold shown generally in FIGURE 7.

FIGURE 9 is a sectional view taken along the line 9-9 of FIGURE 8 with the feed pipes omitted.

FIGURE 10 is a sectional view taken along line 10 -10 of FIGURES.

Referring to the drawings, and particularly to FIGURE 1, I have shown a base 7 on which a vertical casing 8 is mounted. This casing, as is shown in FIGURE 2, includes an outer wall 9 and an inner wall 10, through which a suitable heat exchange medium, e.g., hot oil, steam, Dowtherm or other heat exchangefiuid, is circulated through an inlet, not shown, provided adjacent the bottom of the casing to an outlet, not shown, provided adjacent the top of the casing. Mounted within and closely fitting the casing 8, as appears in FIGURES l, 2 and 6, are two interfolded helical rotors 11 and'12,

mounted respectively upon composite hollow tubes 14 and 16. In place of two rotors, one can use three, four, or more, as desired, these being suitably interleaved.

Also mounted in a passage 20 in the casing 8 is a hollow tube 21 having a plurality of arcuately formed spaced vanes 22, these being disposed annularly about the tube 21 at different elevations, as appears in FIGURE 1. In operation, as will be presently explained further in detail, material can be discharged from between rotors 11 and 12 into passage 20, the material falling down through the passage 20 with the material cascading from one vane to the next as it passes down the casing in heat exchange relation.

At their upper ends, tubes 14 and 16 are supported by stub-shafts 26 and 27 in combination radial and thrust bearings 28 and 29, the hollow rotors hanging in tension from these. Similarly, hollow tube 21 is supported by stub-shaft 30 in the combined radial and thrust bearing 31 provided at the top of the casing, the tube hanging from this bearing. At its lower end, each hollow tube 14 and 16 is supported for rotation by a bearing structure, generally indicated at 32, and including a sealing structure 33 mounted in the housing 34. The sealing structure 33 includes a hollow thimble 36 welded to a flange 38 which is secured on an end of a pipe 37. The end of another pipe 39 is secured to the fiange 38; pipes 37 and 39 make up each of the hollow tube structures 14 and 16. The thimbles 36 are each slidable on shafts 41 and 42, being secured by splines (not shown). An annular groove 35 in the lower end of thimble 36 and an annular ring 40 provide a labyrinth seal about the associated shafts.

Shaft 41 is joined to one end of another shaft 43 by coupling 44, the other end of shaft 43 being carried in a bearing 46 mounted upon the base 7. A sprocket wheel 48 is provided upon shaft 43 and a chain 49 is passed about the sprocket wheel 48 and about a sprocket wheel 51 provided upon the drive shaft 52 extending from an electric motor 53 mounted within the base 7.

Shaft 42 is joined by a coupling 54 to one end of another shaft 56, the latter having one end supported in a bearing 57 provided on base 7. Shaft 56 carries a gear 58 enmeshed with a gear 59 provided upon the drive shaft 52 of the prime mover 53. Shaft 41 also carries a sprocket wheel 61 and a chain 62 is extended about this sprocket wheel and about another sprocket wheel 63 provided upon a shaft 64, the latter having one end supported in bearing 66 on the base 7, while the other end is joined by a coupling 67 to a shaft 68, the latter extending through a thimble 36, which is joined to a pipe 71 at one end. The other end of pipe 71 is joined by a sleeve 72' to the end of tube 21. Couplings 44 and 67 permit removal of the shafts 41, 42 and 68 and the associated drive mechanisms without disturbing the rotors 11 and 12, and tube 21.

The pipe 71 includes a short length of steep pitch multiple lead helical blades, generally indicated at 76, this being disposed in a hopper 77 at the bottom of easing 8 so that material deposited in the hopper is fed into the rotors 11 and 12. The blades act as ploughs to transport the material sideways into the lifting rotors 11 and 12.

Each of the sealing structures 33 is provided in the upper portion of easing 8, this being designated as 81, and being filled with tightly compressed wool 82. In operation, any material which may leak into the wool is absorbed by the lanolin in the wool and forms a homogeneous mass; the wool and labyrinth seal prevent leakage of the gypsum down into the lubricant in the base 7.

Referring particularly to FIGURES 1 and 2, it is to be noted that a movable gate 91 is provided intermediate the ends of the rotors 11 and 12 to permit material to pass from rotors 11 and 12 into the passage 20. To permit adjustment of the gate 91, U-shaped member 92 is provided, fitting about the tube 21. The U-shaped member includes a stem 93, slidably movable in a sleeve 94,

4 the latter being supported by a transverse member 96 in a support casing 97, provided upon the side of the casing 9. A threaded rod 98 extends from the stem 93 and a hand wheel 99 on the threaded rod enables the position of the gate 91 with respect to the opening in the casing 9 to be altered to suit. While only one gate has been provided, more can be utilized if desired.

Material to be discharged passes off the upper end of the rotors 11 and 12 at the top of the casing 9 and is discharged onto a chute 106. A movable extension 107 is hinged as at 108 on the edge of chute 106 and is manipulatable by lever 109 between the full-line position in which it is shown in FIGURE 1 and the dotted line position. In the full line position, the chute serves to discharge material exteriorly of the apparatus for packaging or further processing as may be required. When the chute is tipped into the dotted line position, it serves to return the material for passage downwardly through passage 20 or return selectively into the hopper.

FIGURE 6 illustrates the placement of the inlet 9a and outlet 9b connections for the jacket 9 of FIGURES 1-5.

A modified form of the structure incorporating means for dividing the apparatus into varying temperature zones is seen in FIGURE 7. For clarity of illustration, the flights themselves have been omitted in this drawing. The flight support consists of the two concentric pipes 37 and 39 previously described. Preferably, the exterior pipe' 39 constitutes at least 60% of the total diameter of the screw (including the flight). Sand seal 200 prevents passage of hot gasses between plate 201 and plate 202. Manifold unit 206, shown in greater detail in FIGURE 8, is welded to plate 201. The manifold comprises a pair of concentric tubes 208 and 210 secured together by means of spaced rings 213.

Outer bearing race 214 with inner bearing race 216 holds bearing 218. The inner bearing race fits about ring 220. Bolted to ring 220 is flanged sleeve 224 which supports tube 226. Rings of packing 228 and 230 are positioned about the tube 226 and these are held in position by the cooperation of circular ledges 232 and 234, which are welded to tube 226, housing 236 and packing follower 238. Mounted concentrically within the tube 226 is the tube 240, which is telescoped into the inner concentric 210 of the manifold 206. Mounted about the top of the tube 240 is bronze bushing 242, having an O- ring 244 mounted therein. At the upper end of the housing, packing 246 maintains a tight seal between the housing and valve stem 248, a packing follower 250 being employed in conjunction therewith. Stationary yoke 252 (only a part of which is shown) is tapped for receipt of the threaded upper part of valve stem 248. The yoke may be supported by the housing 236, if desired. Longitudinal movement of the valve stem 248 relative to the yoke 252 by turning wheel 254 is thus possible.

Referring now to the lower portion of the structure and in particular to that portion of the structure which constitutes the means for conveying heated fluid to various temperature zones: joined to the exterior tube 208 of manifold 206 is the top zone feed pipe 260. Returning to the manifold at the topmost portion thereof is top zone return pipe 262. Sealing ring 264 seals the top zone from the center zone, so that hot oil or other fluid may pass downwardly through line 260 and into the topmost zone (as defined by pipes 37 and 39) and thereafter return to the manifold through conduit 262. Conveniently, these pipes are of about 1 diameter. Note that pipe 260 is coiled (as are various of the other pipes to be discussed below) to provide for expansion and contraction under the influence of varying temperatures. A second feed pipe 266 provides means of conveying hot oil to the middle zone and such hot oil is conveyed back to the manifold by means of return pipe 268. Seal 270 divides from the lowermost zone that portion of the area between pipes 37 and 39 which constitutes the middle zone. One inch pipe is conveniently used for supplying and returning heat exchange fluid to the second zone also. The lowermost section is conveniently piped with 1%" pipe since the piping is longer and greater friction is encountered as the oil passes therethrough. Pipe 272 passes directly downwardly from the lowermost end 254 of the manifold and fluid is returned to the manifold by means of pipe 274.

Control of temperature within the zones is accomplished by passing a hot fluid, conveniently hot oil at about 450 F., from an outside source, not shown, into the orifice 276 at the top of housing 236. The oil passes downwardly between the stern 248 and tube 240 and thence into the manifold. The oil continues to pass downwardly through conduit 272. No means need be provided for controlling. flow therethrough, as the unit may be shut down and no oil fed when it is desired to terminate flow into the lowermost, hottest zone. Sleeve 280, which is pivotally secured on the end of valve stem 248,wi1l allow passage of fluid longitudinally thereof, as seen in FIGURE 9. As shown in FIGURE 8, oil is free to flow into each of the individual feed pipes when sleeve 280 has been withdrawn sufliciently to expose both groups of holes 284 and 286 which control flow to the middle and upper sections respectively. sleeve 280 is lowered, holes 286 (of which there are 12 placed in groups of three at 90 angles about tube 210) will be closed, with the result that less oil is allowed to flow through conduit 260 and back through conduit 268. This regulation of the flow rate of the hot fluid in turn regulates the amount of heat which will be transferred through wall 39 to the gypsum itself. As the sleeve 280 is moved farther down, it will obstruct the passage of fluid through holes 284 and thus to decrease the quantity flowing downwardly through conduit 266, and returning through line 268. In any event, hot fluid continues to flow downwardly through conduit 272 and to return through conduit 274; no means need be provided in the manifold for controlling the flow of fluid to the lowermost section since it will be desired to maintain this section at the maximum temperature at all times, except, of course, when the entire machine is shut down.

In the preferred embodiment, the machine consists of three sections, each of which are feet high, each section having an inlet and an outlet as described. It is evident, however, that any number of sections might be provided, and hence the inventive concept embodied herein is not to be restricted to a structure utilizing three temperature zones solely.

A similar arrangement of zones may be provided between jacket 9 and shell 10 by means of seals 300 and 302. Inlet and outlet lines 304 and 306, respectively, are provided for the topmost zone to provide means for conveying hot oil thereinto for accurate temperature control. Inlet and outlet lines 308 and 310, respectively, provide for ingress and egress of hot oil for the intermediate zone, and inlet and outlet lines 312 and 314, respectively, provide means for conveying hot oil into and removing it from the lowermost zone. Valves of conventional structure (not shown) on lines 304 and 308 regulate the quantity of oil flowing through the upper and middle zones.

The structure described herein may be utilized either in a vertical or horizontal application and in any case 'makes it possible for the gypsum or other material being processed to be continually advanced through a single piece of equipment without necessity for a change of direction or heating vessel.

As pointed out earlier, a most etficient system is obtained when the central pipe on which the flights are mounted (pipe 39) constitutes at least 60% and preferably 75% of the total inside diameter of pipe 10. The central pipeshould be large, the depth of the flights It will be seen that as 6 around it relatively narrow, and the'pitch of the fiigh'ts relatively close. This construction is important to the proper functioning of the apparatus.

The use of separate zones as described herein makes it possible for a lower zone to be maintained at a relatively high heat to bring gypsum up to the calcining stage. The central zone may be maintained at a temperature which will hold the gypsum at a calcining temperature for a given period of time, depending on the speed of the screws, and the upper Zone can be cooler so as to drop the temperature of the gypsum just prior to the time it is to be discharged.

Preferably, the structure is stood on end, though it may be used with less advantage in a horizontal manner. When the device is standing upright, the screws run at a substantially greater speed to elevate a given tonnage of material than would be the case if one were conveying the same tonnage horizontally. This is due to the fact that an elevating principle must be utilized rather than merely allowing the material to slide down a hill. The material is transversely thrown against the opposite screw of the opposite hand rotation, which in turn throws it back to the first screw, the material in its elevating path traversing a zigzag path between the bottom inlet and the top discharge. This zigzag path is longer than a straight line of the type which would be described where the device is used while resting horizontally.

As the material traverses this zigzag path, it is scattered over the heated central standards 39 of the screws, as well as the heated casing 10 surrounding the screws. Hence, in the vertical-type heat exchanger, one obtains a. heating effect from. both the casing and the screws, the effect of the casing being maximized due to the fact that there is no tendency for a layer of material to cake upon the wall surface. In the upright embodiment, the casing acts not as a floor but as a wall. As a result, approximately twice to three times the prime heat exchange surface is available in a vertical unit as is available in a horizontal unit having the same screw diameter.

When the device is placed on its side, a screw speed of about 7-10 r.p.m.. may be satisfactory, where if it is stood upright, a speed of between 20 and 25 r.p.m. will be needed.

v In operation, the crude, finely divided rock is fed into the feed hopper. The material is lifted upwardly by the rotors 11 and 12 through the elongated vertical heating zone; the rotors turn relatively slowly, e.g., 25 r.p.m. As the gypsum moves upwardly through this zone, portions are moved transversely while other por-- tions are continuously falling back between the flights and the casing and from off one flight against the hollow pipe onto the other flight. The net result is a copious evolution of steam which fluidizes the rock and causes it to flow much like water, all while a gradual dehydration of the material to the desired hemihydrate form takes place. 'Because of the vertical lift required, the retention of the solid material is controlled without the use of ,Weirs or dams as is required in some gypsum calcining equipment. If desired, depending upon the particular size of apparatus and the load applied, a portion of the material can be returned to the inlet by manipulation of one or more of the gates 91. In practice, a resident time measured in a matter of minutes usually suflices for the production of a calcined material which is of uniform quality and extremely high grade.

invention may be made without departing from the spirit and scope thereof, and therefore only such limitations should be imposed as are indicated in the appended claims.

This application is a continuation-in-part of application Serial No. 667,447, filed June 24, 1957, for Process and Apparatus for Calcining Gypsum, now abandoned.

I claim:

1. A device for processing material to be heated comprising: a first elongated hollow tube having a second hollow tube spaced therefrom and mounted in concentric fashion thereabout; said second tube having a spiral conveyor flight mounted thereabout; means for rotating said tubes; sealing means dividing the space between the said tubes into at least two zones; a source of heated fluid; a manifold fixed substantially entirely within said first elongated tube; means for supplying heated fluid from said source to said manifold and piping for conveying said fluid from said manifold to each of said Zones; means mounted within said manifold for varying the percentage of the total heated fluid directed to any given zone from said manifold; and an external shell surrounding the outermost of said tubes providing a heat exchange zone between the said shell and said outermost tube for passage therethrough of said material being processed.

2. A device for processing material to be heated comprising: a first elongated hollow tube having a second hollow tube spaced therefrom and mounted in concentric fashion thereabout; said second tube having a spiral conveyor flight mounted thereabout; means for rotating said tubes; sealing means for dividing the space between the said tubes into at least two zones; a source of heated fluid; a manifold fixed substantially entirely within said first elongated tube; fluid-conveying means joining said source and said manifold and means for conveying fluid away from said manifold; an inlet conduit and an outlet conduit joining said manifold and each of said zones; means in said manifold for regulating the amount of fluid passing from said manifold to all but one of said inlet conduits; and an external shell closely surrounding the outermost of said tubes and associated flight providing a heat exchange zone between said shell and said outermost tube for passage therethrough of said material being processed.

3. The structure of claim 2 wherein the said hollow tubes are substantially vertical.

4. The structure of claim 2 wherein the said hollow tubes are substantially horizontal.

5. The structure of claim 2 wherein the said exterior shell has a jacket mounted thereabout in spaced relationship thereto, the space between the said shell and the said jacket being divided into zones corresponding in number and position to those between the said hollow tubes, and means for conveying a heated fluid into each of said zones.

6. A device for processing material to be heated comprising: a first elongated hollow tube having a second hollow tube spaced therefrom and mounted in concentric fashion thereabout; said second tube having a spiral conveyor flight mounted thereabout; means for rotating said tubes; sealing means for dividing the space between said tubes into at least two zones; means supplying heated fluid; a manifold fixed substantially entirely within said first elongated tube; means for conveying heated fluid from said supply means to said manifold and piping for conveying said fluid from said manifold to each of said zones; means mounted within said manifold for varying the percentage of the total fluid directed to any one given zone from said manifold; and an external shell surrounding the outermost of said tubes and associated flight providing a heat exchange zone between said shell and said outermost tube for passage therethrough of said material being processed, said second hollow tube having a diameter of at least about 60% of the diameter of the said external shell.

' 7. A device for processing material to be heated comprising: a first elongated hollow tube having a second hollow tube spaced therefrom and mounted in concentric fashion thereabout; said second tube having a spiral conveyor flight mounted thereabout; means for rotating said tubes; sealing means for dividing the space between the said tubes into three zones; a source of heated fluid having a conduit for conveying said heated fluid therefrom; a manifold mounted on one end of said conduit, said manifold being fixed to one end of said first hollow tube; lines joining said manifold and each of said zones for conveying said fluid from said manifold to each of said zones and back from each of said zones to said manifold; means integral with said manifold for controlling the rate of flow of said fluid to two of said zones, the remaining zone being the zone adjacent one end of said hollow tubes; and an external shell surrounding the outermost of said tubes and associated flight pro viding a heat exchange zone between said shell and said outermost tube for passage therethrough of said material being processed.

8. The structure of claim 7 wherein said second hollow tube has a diameter of at least about 60% of the diameter of the said shell.

9; The structure of claim 7 wherein the said shell has provided thereabout a jacket, the space between the said shell and the said jacket being divided into three zones corresponding in position and length to the zones formed between the said tubes; and means for passing heated fluid into each of said zones between said shell and said jacket.

10. A device for processing material to be heated comprising: a first elongated hollow tube having a second elongated hollow tube spaced therefrom and mounted in concentric fashion thereabout; means for rotating said tubes simultaneously; sealing means dividing the volume between said tubes into at least a first and second zone; a source of heated fluid; a manifold fixed at one end of said first elongated tube, said manifold comprising a pair of short hollow tubes mounted in concentric fashion and having a sealing ring intermediate the ends thereof holding said short concentric tubes in a fixed relationship and dividing the volume between the two short tubes into a fluid charging zone and a fluid return zone; at least a single hole entirely on the charging zone side of the said sealing ring passing through the wall of the internal short tube; a sleeve mounted within the internal tube and adapted to slide longitudinally thereof and of suflicient size to cover the said hole entirely; two conduits on the return zone side of the sealing ring communicating with the return zone, said conduits providing communication with the internal volume of each of the two zones defined by the elongated hollow tubes and sealing ring therefor; a conduit on the charging zone side of the manifold providing communication with the charging zone and with the interior of the first zone formed by the elongated tubes and sealing ring therefor; another conduit communicating directly with the interior of the inner short concentric tube and with the second zone defined by said elongated tubes and sealing ring; manually operated means for moving the said sleeve 1ongitudinally of the internal zone a suflicient distance to cover and uncover the said hole therein; means for conveying a heated fluid from the source thereof to the interior of the said short internal concentric tube and means for returning the said fluid from the return zone of the said manifold to the said fluid source; and an external shell surrounding the outermost of said elongated tubes providing a heat exchange zone between the said shell and the said outermost of said elongated tubes for passage therethrough of said material to be processed.

11. The structure of claim 10 wherein additional sealing means are provided for subdividing the manifold charging zone into two separate zones and wherein at least a single hole provides communication between the volume within the innermost of said short tubes and each of said separate charging zones, wherein the said sleeve is of sufiicient length to cover all of said holes simultaneously, wherein an additional sealing means provides a third zone between the elongated concentric tubes, wherein conduits communicate with the interior of each of said manifold charging zones and two of said zones formed between the said elongated tubes and wherein additional conduit means provide communication between the said two zones between said elongated tubes and the discharge zone of the said manifold.

References Cited in the file of this patent UNITED STATES PATENTS Rowland Aug. 11, 1896 -Duttweiler Mar. 5, 1935 Loomis Jan. 7, 1936 Barrow June 30, 1936 Buecken et al. Feb. 13, 1951 Buecken June 17, 1952 

